ceph: use timespec64 for inode timestamp

Since the vfs structures are all using timespec64, we can now
change the internal representation, using ceph_encode_timespec64 and
ceph_decode_timespec64.

In case of ceph_aux_inode however, we need to avoid doing a memcmp()
on uninitialized padding data, so the members of the i_mtime field get
copied individually into 64-bit integers.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>